This application claims priority to and the benefit of Korean Patent Application No. 10-2004-0092809, filed on Nov. 13, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a bipolar plate and a direct liquid feed fuel cell stack comprising the same.
2. Description of the Background
A direct liquid feed fuel cell generates electrical power by electrochemical reactions between an organic fuel such as methanol or ethanol and an oxidant such as oxygen. The fuel cell has high energy density and high power density. Since the direct liquid feed fuel cell uses the fuel directly, external peripheral devices such as a fuel reformer are not required and the fuel can be easily stored and supplied.
As illustrated in FIG. 1, a single direct liquid feed fuel cell includes a membrane-electrode-assembly (MEA) structure having an electrolyte membrane 1 interposed between an anode 2 and a cathode 3. The anode 2 and the cathode 3 include fuel diffusion layers 22 and 32 for supply and diffusion of fuel, catalyst layers 21 and 31 to aid an oxidation-reduction reaction of fuel at the electrodes, and electrode supporting layers 23 and 33, respectively.
The catalyst layers 21 and 31 may comprise a noble metal such as Pt that has excellent electrochemical properties even at low temperatures. To prevent catalytic poisoning due to carbon monoxide in the reaction byproducts, alloys that include transition metals such as Ru, Rh, Os, or Ni may also be used. The electrode supporting layers 23 and 33 are made of carbon paper or carbon cloth and their surfaces are wet-proofed for easy supply of fuel and discharge of reaction products. The electrolyte membrane 1 may be a polymer membrane with a thickness of 50 μm to 200 μm. A proton exchange membrane that retains moisture and has ionic conductivity is usually used as the electrolyte membrane 1.
A direct methanol fuel cell (DMFC) operates by an electrochemical reaction between methanol and water. At an anode reaction, fuel is oxidized and at a cathode reaction, oxygen is reduced by protons and electrons. The reactions are as follows:Anode Reaction: CH3OH+H2O→CO2+6H++6e−Cathode Reaction: 3/2O2+6H++6e−→3H2OOverall Reaction: CH3OH+3/2O2→2H2O+CO2 
Methanol reacts with water at the anode 2 to produce carbon dioxide, protons, and electrons. The produced protons migrate to the cathode 3 through the electrolyte membrane 1, which can be a proton exchange membrane, and react with oxygen and electrons, which are supplied via an external circuit, at the cathode 3 to produce water. Overall, in the DMFC, water and carbon dioxide are produced through the reaction of methanol with oxygen.
The theoretical voltage generated in a DMFC single cell is approximately 1.2 V. However, the open circuit voltage at room temperature and atmospheric pressure is 1 V or less and the actual operating voltage is approximately 0.4 V to 0.6 V because there is a voltage drop due to activation overpotential and resistance overpotential. Thus, to obtain a desirably high voltage, several single cells are connected in series.
A stacked cell is obtained by stacking several single cells that are connected in series. In this configuration, a conductive bipolar plate 4 is interposed between single cells to electrically connect adjacent single cells.
A graphite block that has good electric conductivity, mechanical strength, and machining properties is usually used as the bipolar plate 4. A block of a composite material comprising metal or a conductive polymer may also be used. Flow channels 41 and 42 that separately supply fuel (methanol) and air, respectively, to the anode 2 and cathode 3 are formed on both sides of the bipolar plate 4. The bipolar plate 4 is positioned in the middle of the stack between adjacent single cells, and end plates (not shown) which are monopolar plates that supply fuel or oxygen to the electrodes 2 or 3, are disposed at ends of the stack. Flow channels 41 and 42 that supply air and fuel to adjacent single cells are formed in the end plates.
FIG. 2 is a plan view of the bipolar plate in which liquid fuel channels, for example, are formed.
As shown in FIG. 2, in the conventional bipolar plate 4, a plurality of serpentine fuel flow channels 41 are formed in an electrode region 47 in which an MEA is disposed. Outside the electrode region 47, a manifold 46 connected to entrances and exits of the fuel flow channels 41 and fuel pathway holes 43a, 43b, 44a, and 44b penetrating the bipolar plate 4 are formed. The fuel pathway holes 43a, 43b, 44a, and 44b are connected to the manifold 46 to supply or discharge the liquid fuel or an oxidant. That is, the fuel pathway holes 43a and 43b, respectively, act as an inlet and outlet for the liquid fuel and the fuel pathway holes 44a and 44b, respectively, act as an inlet and outlet for the oxidant.
The serpentine flow channel illustrated in FIG. 2 and the fuel channels disclosed in U.S. Pat. Nos. 6,309,773 and 6,099,984 have a bending angle of 90°, which causes a large pressure loss of the fluid in the fuel flow channels 41.
In addition, U.S. Pat. Nos. 6,541,145 and 6,586,128 disclose flow channels formed by islands, and liquid or air flows in a space between islands. Such a structure reduces the size of the water droplets and bubbles that are discharged at an anode and a cathode, which facilitates fluid flow. However, it is difficult for these flow channels to uniformly supply fuel to an MEA due to their small relative areas.